


Undertale headcannons, AUs and ideas that keep me up at night

by Trashcanraccoon



Category: Undertale (Video Game)
Genre: Alternate Universe - Horrortale (Undertale), Alternate Universe - Swapfell (Undertale), Alternate Universe - Underfell (Undertale), Alternate Universe - Underswap (Undertale), Anxiety, Bara Sans (Undertale), Body Image, Brain Fog, Chronic Illness, Chronic Pain, Chubby Reader, Dead Dove: Do Not Eat, Depression, Disability, Drabbles, Fluff and Angst, Frisk's prounouns are They/Them, Gen, Help, Horrortale Papyrus (Undertale), Horrortale Sans (Undertale), Hurt/Comfort, I'm Bad At Tagging, Implied/Referenced Child Abuse, Maybe - Freeform, My First Fanfic, No Frans - Freeform, Non-Verbal Frisk (Undertale), Nonbinary Frisk (Undertale), Other Additional Tags to Be Added, Panic Attacks, Parent Sans (Undertale), Post-Undertale Pacifist Route, Post-Undertale Pacifist Route - "I want to stay with you.", Protective Sans (Undertale), Reader Is Not Frisk (Undertale), Reader has serious issues, Reader-Insert, Recovery, References to Depression, References to Undertale Genocide Route, Sans (Undertale) Has Issues, Sans (Undertale) Needs a Hug, Sans (Undertale) Remembers Resets, Sans is flawed, Sans makes mistakes, Spoilers - Undertale Genocide Route, Suicidal Thoughts, Swapfell Papyrus (Undertale), Swapfell Sans (Undertale), Therapy Positive, Underfell Papyrus (Undertale), Underfell Sans (Undertale), Underswap Papyrus (Undertale), Underswap Sans (Undertale), Undertale Spoilers, Worried Sans (Undertale), body issues, but they end up getting help, ideas that I might write, im so sorry, no betas we die like men, no sanscest, not enough of that in fics, plus size reader, reader has a chronic illness, reader is female, seriously go get some help guys pls, short and sweet, this is a mess
Language: English
Status: In-Progress
Published: 2020-04-27
Updated: 2020-04-27
Packaged: 2021-03-02 00:42:09
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 2
Words: 3,441
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23876365
Author URL: https://archiveofourown.org/users/Trashcanraccoon/pseuds/Trashcanraccoon
Summary: Sometimes I have ideas and write them on tumblr but then realise that they are really long and no one reads them on tumblr anyway so for the first time since I was, like, 13 or something I'm doing fanfic....What else was I gonna do during quarantine? University work? Ha!Literally a bunch of drabbles and ideas around Undertale and Undertale AUs. Trigger warnings in the tags, please read and keep yourself safe. If you like anything here then feel free to dig around in the sandbox with credit :)Why would Frisk/reader go to mount Ebott knowing the legends that no one returns?What if Frisk had a more permanent reminder of the genocide run?
Relationships: Alphys/Undyne (Undertale), Papyrus (Undertale)/Reader, Sans (Undertale)/Reader
Comments: 3
Kudos: 24





	1. Legends say that those who climb the mountain never return...

**Author's Note:**

> I've not wrote fanfiction since I was in my early teens and even then I was really dumb and didn't really understand that taking bits and pieces of other people's work was, you know, stealing. I don't even know if I'm good or not but I've felt compelled lately to write, especially since where I live is in lockdown. If there's useful critique you have about any of these feel free to pull me up for it but please be nice cause I will cry. Also they are all just drabbles basically so I've not done a lot to make sure it's anything other than spelled right and readable. I might write one or two of them but if you like the idea and have your own thoughts on it then feel free to play around with it, just credit me if you're posting it here or anywhere else :)
> 
> This started on tumblr @Caitlyn-comics (my art tumblr) as a rant sketch inspired by work by @insertdisc5. I wasn't feeling great and it’s always sat with me that these children climbed a big ass mountain without adult supervision. Especially Frisk. Some headcannons for a kinda mental health AU? I don't know, it was therapeutic to write. I talk about anxiety, depression and chronic illnesses and disabilities. I have fibromyalgia so it was important to me to include that, especially because I don't see a lot of that mentioned in fics outside of Horrortale Sans and Papyrus. I've also used neuroatypical language like hyperfixations and info dumping for the same reason of inclusion, plus the idea of coping mechanisms. I recently read "Babysitting Blues" by wannabuyahotcat and I kinda like the idea of Blue being on the spectrum but what I know about Autism is from my non verbal brother so I'm iffy about committing to that. It's all very self indulgent. I actually started writing an actual story from this idea so maybe I'll get that put up soon. Sorry this part is long!

The fact that Mount Ebott has legends about people who climb the mountain never returning is a big part of Undertale and a lot of the fan ideas and this came from thoughts of why would anyone then climb the mountain unless a) they didn't believe in the myths b) it was a dare or c) they believed in the legend and wanted to disappear. 

It makes me think of a sort of AU were MC/reader/Frisk purposefully goes to the mountain because of the rumours and obviously isn’t successful but meets a bunch of monsters who know why they came to the mountain and want to help. It can of course focus on one AU with all the main characters but it was fun to think of all the Sans and Papyrus from the different AUs I like.

**Classic Sans:** knows what it’s like to feel hopeless, to not want to get out of bed, how some things become so difficult to achieve. Knows what it’s like to have to hide how you feel, be happy and funny to make sure others don’t worry. If you need to do the thing, he will do the thing with you being “helpful” with his puns. He’s always there with soft encouragement and always praises the small things. If you have a disability or chronic illness that makes you tired or makes it hard to walk then, well, he'll take any excuse for a shortcut to be honest. Will probably make you those star stickers; _“you got out of bed!”_ , _“you brushed your teeth!”_ , _“you managed social interaction!”_ , _“you didn’t cry today!”_. Is interested in what you want to do in education and/or work. Will help you plan and encourage you to work towards whatever that is that makes you happy. He knows some folk that would be interested in you working with them. 

**Classic Papyrus:** Obviously is your personal cheer squad. Knows just how to motivate you but if it isn’t working and it’s a bad day knows when to calm down and let you take things at your own pace. Tell him your deepest desires, your biggest dreams, no matter how unattainable or silly it seems. You want to be an astronaut? You need to be physically healthy, go to school to get good grades and do training to achieve that? Pffft, that’s only three things, you can totally do that! Teaches you how to hold onto a dream and keep back the bad thoughts that say you can’t do it. You’ll show them! Will cook and bake with you while making some idle chatter. Afterwards will sit with you and watch MTT or something you’re currently interested in cuddled under a blanket. The king of early AM or late PM supermarket trips which can be fun because you end up having the strangest conversations, like what kind of shape you think each other is. Will probably engage in antics too because his energy is so susceptible. 

**Fell Sans (Red):** the worst kept secret is that he is a softie. Will chill out with you and give you as much physical touch as you’re comfortable with. Little thumb strokes, hair strokes, hip bumps, squeezes and full on “you are now my personal hugs pillow, now chill out and watch the tv show”. Is your protector, there behind you to make sure no one walks over you or just there as silent encouragement when you need to do scary adult things. He's also just a good guy to hang out with and have a laugh. Knows what it’s like to have manic lows were the sadness is energetic and wants out. Knows how hard it is to stay strong when you need to be supported. Is there for you when this happens, even if he just silently holds you.

**Fell Papyrus (Edge)** : You are the only one he is soft and gentle to. You know what that means? You are worthy of the terrible Papyrus! These other humans are merely jealous of your brilliance! Actual fashion icon who encourages you to express yourself however you want. Cut or dye your hair? Edge is there critiquing cuts and colours and of course approving of your choices. Your style is very much worthy of the terrible Papyrus’s approval. Humans are staring? Yes, how could they not? You look amazing! And the commoners are weak to resisting such style! Will absolutely call you by your preferred pronouns or name without question, although all skeles will do so too. Will probably accompany you everywhere and honestly, you'll probably have to get used to a squad of skeletons following you around unless you ask them not to. Teaches you and helps you to be comfortable with yourself and that you are worth your own existence. 

**Swap Sans (Blue):** Another very energetic cheer squad! Preaches the importance of routine and activities. Won’t push you with exercise if you aren’t into that but encourages gentle jogs or walks or swimming, anything calming and enjoyable. After all, what’s the point if you don’t enjoy it! Swimming is particularly good for you if you suffer with chronic pain or mobility issues because it's low impact and easy on your joints. He'll join in and takes splash wars very seriously. God help you if there is a slide, you'll be there for hours. May or may not wear a floatie to two... Will help you figure out what the best times are you to do things, are you an early bird or a night owl? When do you get hungry? Do you have medication? When do you need to take them? He makes a very cool schedule with reminders on your phone so you don’t forget. Is very interested in your hyperfixations and interests and is eager to learn all about it! Info dump your little heart out! Blue won’t judge and is 9 times out of 10 more likely to want to become involved too! He secretly knows what it’s like to not be respected for being yourself, whether that be for the things you love or how you look. He’s very well aware of what other monsters see him as. But! He will show you how the magnificent Sans achieves what he wants, does what he wants all the while being true to himself!

**Swap Papyrus (Stretch):** Another chill boy. He’s always there for you if you need to chill, talk or be distracted. No, literally. He always seems to be close to you wherever you are and whatever you’re doing, without actually moving at all... Is not at all a therapist but will listen to you when you need to talk or vent or explain something you love or have just learned. Pull up a beanbag, pal, tell him all about it. Will go with you to any doctors appointments or therapy sessions and be there for you, slouched down in his chair in the waiting room. Very good at distracting you with music or games or some sweet treats. Sometimes you'll find things left outside your bedroom door or hidden in your room that you swear wasn't there before. Will find you chilling out somewhere and make a big deal of climbing up behind you and laying his chin on top of your head. No one knows what that's about but just let him, it's comfy and it makes him happy and gets you out of doing the dishes because he always falls asleep like that. A rock of a skeleton boy. Knows what it’s like to be so overwhelmed by your feelings being trapped inside you that you loose interest in things and become numb. Will show you coping mechanisms he learned himself and will always never let you forget that he is always there for you.

**SwapFell Sans (Blackberry):** This is a very smart boy. He clawed his way up to royal guard status without the usual violence, he doesn’t need to. Is an expert and obsessed with cheating the system and getting clever sneaky payback. Any trouble and he has it sorted, he’s both better and worse than a Karen. Will talk to a manager, teacher, professional and come out the victor. The doctor who doesn't believe your symptoms? Suddenly you have an appointment with a expert. The therapist that was rude and dismissed your feelings? He got a very strongly worded letter and you got an appointment with a much better therapist. That person who bullied you? Strangely bolts whenever you are in their proximity. That random Karen in the supermarket who took a disabled parking space without a badge and made remarks about you not looking disabled? Why did she flinch at the date February 21st 2015 and how does Black know about it? Teaches you what you can do to help yourself in certain situations and how to come across confident in yourself even when you’re not. Somehow knows everything about everyone and is always doing things like reminding you that your medication is running low, you need to book another doctors appointment, even tells you that that thing you were eyeing up is much cheaper elsewhere and he’s already ordered it for you so don’t worry about it. His literal motto is “you don’t need to worry about it anymore, I’ll handle it.” But he knows when to slow down and teach you how to do things so you can do it yourself. He is also a master at pet names. Darling? Sweetheart? Love? Baby? His Queen? He’ll find out what you prefer (i.e blush the most at) and uses it to his advantage, and you’ll love it. _ **Hand kisses.**_ Knows how things can build and build and how sometimes the littlest thing can be the domino effect that makes everything crash. Shows you how to prioritise and take it one task at a time. 

**Swapfell Papyrus (Russ):** Just...a sweet boy. A little nasty, but sweet and a total dork. Why do you sometimes have the urge to protect him and then suddenly feel submissive to his protection? Is a firm believer in therapy through action. You like art? He’s found this little group where you just be calm and do art and maybe talk about your feelings if you want, lets both go. Is music your thing? Here, if you’re sad how about you press all the low keys on the piano at once? **_BOOOOOAAAAAMMMMM_**. Writing? Writing how you feel is very important to both understand yourself and how you’re feeling, to be able to reflect and point out things that triggered you and also be able to show a professional symptoms and explain how you feel without having to strain to remember. But you can also write for the pure enjoyment of words and the movement of your pen or the rhythmic click of your keyboard. Will help you with prompts and inspirations. Will never judge you for using a fandom or interest to help you get through this. Will definitely show you some of his own stuff. Will always like and repost your social media posts online for everyone to see. Is an expert nest builder, fort architect and burrito roller. Always has the softest blankets and fluffiest pillows. Knows how lonely you can feel even when surrounded by people, even when it doesn’t make sense to feel lonely or sad. Teaches you that it’s ok to feel these emotions but shows you when you need to stop in order not to become consumed by it. 

**Horror Sans (Mars):** Is deeply knowledgeable about trauma. He had a long road recovering from his. His trauma might make you feel stupid for your feelings but he will shut that down immediately. His broken leg doesn’t make your broken arm any less painful and that counts with everyone else, don't undermine your own troubles. Will be able to understand how you feel about your trauma and how that has affected you. He’s very careful to put down boundaries so neither of you trigger the other. He’s your mental health buddy, someone close who’s going through the same thing that you can talk to and know that you are fully understood. You can ask questions to help understand yourself and your own journey. Especially close because you both have chronic pain and brain fog. Doesn't judge the amount of times you ask him to repeat himself because you heard him but you brain didn't understand what the words meant, or how many times you'll be talking about something and suddenly forget what you were talking about. You're both always confusing each others notebooks with things you need to remember with each others. Probably mixed up each other's appointments. Will show you how to focus and enjoy small things. Believes that people are either water people or earth people. Does the sound and sight of water make you calm? Just the rhythmic movement and sound drift you away while you both pick up cool looking rocks and smooth sea glass and sometimes even a shell! Or do you like big grassy parks or forests you can walk to your hearts content and just breathe? You both pick up some cool leaves to press between a book, take some pictures of plants and trees and sunsets. Whenever you need a break, whether you realise or not, he will whisk you away to just bask in silence for a while. A big cuddler and is known for just picking you up and carrying you around (if you like that). He has a permanent blanket nest, hop on in and have a nap. 

**Horror Papyrus (Sweetie):** Knows deeply how hard it is to adapt sometimes, and how adapting to situations can hurt you later. Knows what it’s like to be judged by how you look, especially if there are things you need to hide. Helps you with Edge’s quest, especially with clothes. He’s very good at finding unique objects that no one else has. Often comes back from thrift and charity shops with a few things he think’s you’ll like. Will cuddle up with you and listen to how you feel about yourself and the way you look. Helps you come to terms with yourself, accept yourself. Always has a compliment for you, building it up from small things so you can learn to believe and accept them. Knows ways to help you if you sometimes dissociate and especially if it’s triggered by photos or mirrors. If you need to go to the dentist or the doctor for operations or procedures, he’s right there with you holding your hand, he remembers how scary it was the first time he went to a dentist and understands if you still get a little scared sometimes. He himself has some very cool braces for his teeth, some funky glasses and some support for his posture. If you have anything you use for support like a walking aid or wheelchair, he'll help you find things to decorate it. When you're comfortable enough to trust him to show him things you keep a secret that help you calm down, like stuffed toys or fidget toys or hobbies most people think are weird he will sit and excitably listen to you explain and show you his own special things. Loves to teach you how to solve puzzles and will sit behind you and point out words he finds in the word searches. A very good bonding buddy. Also, he always has snacks, he is a snack mom. Sometimes he even slips food and water into your bag and pockets without you realising. 


	2. Genocide Route Scars

**Summary for the Chapter:**

> Sans spots a scar on Frisk's stomach and everyone wonders how they got it.

**Notes for the Chapter:**

> Genocide route spoilers and trigger warning for mentions of abuse, implied suicide and referenced violence. Big Dead Dove: Do Not Eat. It isn't graphic but keep yourself safe.
> 
> Please don't take this as Frans, I didn't intend it to be read like that and Frisk is still young enough that getting help dressing and bathing isn't unusual so please don't go there. Sans is clearly a parent figure here. I didn't write it as Soriel but can be read like that too if you want.

Frisk who does a couple of resets, gets possessed by Chara and does a few genocide runs but ultimately achieves true pacifist and rebuilds their relationship with Sans to the point where he is a parental figure outside of Toriel.

One day Sans is helping Frisk and spots a scar on their stomach....

All Frisks friends had a some point sat down and talked about why Frisk would climb the mountain when there were so many stories saying that those who climbed the mountain never returned. Talked about how Frisk doesn’t talk but maybe has the ability to and worries if it is a result of trauma. Maybe they had faced a situation were unfit parents had came out to claim Frisk and they had made it clear they didn’t want to live with them. Basically, they all worry that Frisk had been in some way abused which lead to them climbing the mountain. 

So Sans is like _shit_ and tries to get a better look by helping them at bath-time or getting ready for bed or school (normal _parent_ things) but Frisk...isn’t having it. And the more Sans tries the more secretive Frisk gets. Toriel knows nothing about this, assumed that Frisk was just independent and didn't want help with these things and so has never noticed any scars, and is upset about it. Everyone gets worried and tries to find out for themselves which only makes Frisk retreat into themselves more. 

So Sans faces the fact that they have been through a lot and out of everyone he is the only one truly close enough to the kid to be able to approach this truthfully. So maybe he sits her down privately and admits to what he’s seen and expresses how everyone is worried about them and wants to help. Frisk is clearly upset and doesn’t want to talk about it, probably starts tearing up, but ultimately stands up and lets Sans help them pull off their jumper...

They have a number of scars; old but still a little red, not healed enough to have gotten to that silvery stage; and they are all overlapping and different sizes leading from her chest down her stomach. Sans is horrified but is struck by how uniform the scars are, how all of them have the same circular look about them, they don’t look like a stab wound but are clearly not burns. It’s not until Frisk turns around and he sees the identical scars on their back matching the positions of the scars on their front that he realises... 

They are scars from the genocide run, when during their fight he had offered them mercy and they had hugged...and then he killed them. 

He’s overcome with horror and disgust. How did he not notice this sooner? But more importantly, I think this is the first time he has had to look into his own actions during genocide run, how he disrespected the **_spare_** ideology in the underground by using it to trick them to get his own revenge. He thinks about how deeply Toriel and Papyrus believes in **_spare_** and how if anyone uses **_spare_** in a fight it means they _don’t want to fight_ , how if spare is used then you _have to_ spare in return. And because of his actions, while they might have been understandable, he has scarred _a child_. 

I can imagine him crying silently at this point. He probably tries to heal Frisk but for some reason they won’t heal and he thinks of how Toriel will be so angry and how disappointed Papyrus will be if he were to admit to them so that they could help heal Frisk but Frisk puts her hands on his cheek bones and just smiles at him tearfully. They forgive him. And Sans would probably argue with them that it isn’t a punishment they deserve, they had no control during genocide run he knows that now, but they both come to the agreement that they both have to accept that this is how it is. 

Sans returns to everyone and tells them that he’s spoke to the kid and it’s nothing they need to worry about. Toriel probably asks what happened that they have the scars but Sans puts his foot down, it’s not his story to tell and he promised it would be a secret. Frisk doesn’t want anyone to know, doesn’t want anyone to worry or look at them in pity and they should all respect that.

So everything pretty much goes back to normal except Sans puts in more effort when it comes to Frisk and soon gets the reputation of pretty much never saying no to the kid and spoiling them. Everyone is a little confused as to where this energy has come from but when Papyrus beams and praises Sans everyone can admit that it’s hardly a bad thing.

Frisk grows up to have an unbreakable bond with Sans and tells him everything without fear of judgement.


End file.
